1. Field of the Invention
The present invention relates to a process for preparing alkoxysilanes which are low in acidic chloride or essentially free from acidic chloride.
2. Discussion of the Background
It is known that alkoxysilanes can be prepared by reacting the corresponding chlorosilanes with an alcohol in accordance, for example, with DE 28 00 017 C2 or EP 0 107 765 P2.
A concern in the course of such procedures is generally to maximize esterification and obtain a good product yield. The survival of unreacted fractions of chlorosilane and hydrogen chloride in the form of what are known as hydrolyzable or acidic chlorine compounds--acidic chloride for short--in the product is not wanted. Within the group of organoalkoxysilanes, it is generally only the aminoalkylalkoxysilanes that include not only acidic chloride but also residues of nonhydrolyzable chlorine compounds, such as chloroalkylalkoxysilane, since such compounds are starting compounds in the preparation of the aminoalkylalkoxysilanes; cf. EP 0 741 137 A1. For the present invention, aminoalkylalkoxysilanes are excluded from the group of alkoxysilanes.
In the light of present-day applications of alkoxysilanes, such as, for example, as adhesion promoters, for hydrophobicization in building protection products, for crosslinking plastics, for modifying surfaces, in glass fiber sizes, for example, and as starting materials for further reaction stages, to name but a few applications, it is therefore necessary to provide products having a very low content of acidic chloride.
In the text below any reference to a chloride-free alkoxysilane is to a product whose acidic chloride content is less than 10 ppm by weight--i.e., which is essentially free from acidic chloride. The detection limit for determining acidic chloride in alkoxysilanes is currently &lt;1 ppm by weight (as determined by argentometric titration in anhydrous acidic solution with potentiometric endpoint determination--AN-SAA-0411).
EP 0 223 210 A2 discloses a method of purifying alkoxysilanes comprising hydrolyzable and nonhydrolyzable chlorine compounds, in which the alkoxysilane is heated in the presence of acid clay or a heavy metal halide, then brought into contact with a neutralizing agent, such as metallic sodium, metallic calcium, alkali metal hydroxide, sodium carbonate, magnesium oxide, alkali metal alcoholate, ammonia, organic amines, alkylene oxides or ortho esters, and separated from the other components by means, for example, of filtration or distillation. In addition, the filter residues arising from this method must be washed with solvent, in a laborius procedure, to remove all silane before they can be disposed of as special waste, or must be worked up and recycled, in a likewise laborious and costly procedure. The heating of said alkoxysilanes in the presence of acid clay or a heavy metal halide is also laborious and can result in unwanted secondary reactions. It is known, for instance, that when chloroalkylalkoxysilanes are heated in the presence of a metal halide, such as iron chloride, aluminum chloride and copper chloride, to name but a few, they are broken down in a process which is accompanied by the liberation of HCl and the formation of alkenylalkoxysilanes. Product discoloration, the formation of mixed esters, and condensation reactions are also observed. Furthermore, owing to traces of the heavy metal halide that is employed, alkoxysilanes obtained by such a method do not in general meet the stringent requirements of the food industry in respect, for example, of plastic packaging or pipes for drinking-water.
DE-A 25 21 399 reveals a process for preparing arninoalkylsilanes in which a reaction mixture obtained by arninating a chloroalkylalkoxysilane is admixed prior to work up with an amount of metal alcoholate equivalent to the amount of chloroalkylsilane and/or chloride present in the mixture. This preparation process makes use of solvents or diluents, such as toluene, hexane or alcohol.
EP 0 282 846 A2 discloses a process for preparing alkoxysilanes with a low content of chlorine compounds by stepwise esterification of chlorosilanes with alcohols in the liquid phase, with removal of the resulting hydrogen chloride. The alkoxysilanes obtained in this way, which still contain a low level of chlorine compounds, are reacted with metal alcoholate--which based on the proportion of chlorine compounds is added in a stoichiometric excess--at a temperature in the range from 80 to 200.degree. C. in the presence or absence of a solvent, such as toluene or xylene, and the alkoxysilane is separated by filtration, for example, from the salts that have been formed. With this process, the long reaction times at relatively high operating temperatures are not very advantageous.
In addition, EP 0 486 279 B1 discloses a process comparable to the abovementioned processes which is intended to remove acidic impurities from alkoxysilanes and which uses, as a neutralizing agent, the metal salt of a sterically hindered amine or an alkali metal alcoholate of a sterically hindered alcohol at a temperature of up to 80.degree. C. over 1 to 2 hours, the neutralized alkoxysilane being distilled. Neutralizing agents of this kind are not available in sufficient quantities for the industrial scale and the separate additional preparation of the neutralizing agent would be complex, costly and therefore ruled out on economic grounds.
EP 0 532 872 B1 likewise discloses a process for preparing alkoxysilanes that are contaminated with hydrolyzable chlorine atoms, where the alkoxysilanes are reacted with an alcohol in a pressure reactor in the presence or absence of excess amounts of a neutralizing agent, reaction taking place at above the boiling point of the alcohol used and under the autogenous pressure, and the salt produced--if appropriate--is separated off and the excess alcohol is removed from the product by distillation. Neutralizing agents disclosed in this case include ammonia, organic amines and sodium alcoholates. It is known that under the abovementioned conditions chloroalkylalkoxysilanes, in particular, are able to react with ammonia or with organic amines to form aminoalkylalkoxysilanes or, in the presence of alcohols and with alcoholates, to form alkyloxyalkylalkoxysilanes.
EP 0 563 883 B1 discloses a process for neutralizing acidic halides in alkoxysilanes which provides for the alkoxysilane to be brought into contact first with a metal alcoholate as a base, which, based on the content of the acidic halide, is added in a stoichiometric excess, and subsequently with an acidic salt which, based on the proportion of the residual base present in excess in the alkoxysilane, is employed in a stoichiometric excess. Here too, the salts are separated by filtration from the product, which can if desired be purified further by stripping off residual alcohol or by distillation. In addition, EP 0 563 883 B1 refers to problems in terms of the color quality of alkoxysilanes, these problems being observed during the work up, in particular during the neutralization and subsequent work up of the alkoxysilanes.